1. Field of the Invention
This invention relates generally to collector optics for a light source and, more particularly, to high efficiency collector optics for a laser plasma extreme ultraviolet (EUV) radiation source that collects a high percentage of the generated EUV radiation.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current trend for photolithography light sources is to develop a system that generates light in the extreme ultraviolet (EUV) or soft X-ray wavelengths.
Various devices are the known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Argon and Krypton, and combinations of gases, are also known for the laser target material. In the known EUV radiation sources based on laser produced plasmas (LPP), the gas is typically cryogenically cooled in a nozzle to a liquid state, and then forced through an orifice or other nozzle opening into a vacuum process chamber as a continuous liquid stream or filament. The liquid target material rapidly freezes in the vacuum environment to become a frozen target stream. Cryogenically cooled target materials, which are gases at room temperature, are required because they do not condense on the source optics, and because they produce minimal by-products that have to be evacuated by the process chamber. In some designs, the nozzle is agitated so that the target material is emitted from the nozzle as a stream of liquid droplets having a certain diameter (30–100 μm) and a predetermined droplet spacing.
The target stream is radiated by high-power laser beam pulses, typically from an Nd:YAG laser, that heat the target material to produce a high temperature plasma which emits the EUV radiation. The frequency of the laser beam pulses is application specific and depends on a variety of factors. The laser beam pulses must have a certain intensity at the target area in order to provide enough heat to generate the plasma. Typical pulse durations are 5–30 ns, and a typical pulse intensity is in the range of 5×1010–5×1012 W/cm2.
FIG. 1 is a plan view of an EUV radiation source 10 of the type discussed above including a nozzle 12 having a target material storage chamber 14 that stores a suitable target material, such as Xenon, under pressure. A heat exchanger or condenser is provided in the chamber 14 that cryogenically cools the target material to a liquid state. The liquid target material is forced through a narrowed throat portion or capillary tube 16 of the nozzle 12 to be emitted under pressure as a filament or stream 18 into a vacuum process chamber 26 towards a target area 20. The liquid target material will quickly freeze in the vacuum environment to form a solid filament of the target material as it propagates towards the target area 20. The vacuum environment in combination with the vapor pressure of the target material will cause the frozen target material to eventually break up into frozen target fragments, depending on the distance that the stream 18 travels and other factors.
A laser beam 22 from a laser source 24 is directed towards the target area 20 in the process chamber 26 to vaporize the target material filament. The heat from the laser beam 22 causes the target material to generate a plasma 30 that radiates EUV radiation 32. The EUV radiation 32 is collected by collector optics 34 and is directed to the circuit (not shown) being patterned, or other system using the EUV radiation 32. The collector optics 34 can have any shape suitable for the purposes of collecting and directing the radiation 32, such as an elliptical shape. In this design, the laser beam 22 propagates through an opening 36 in the collector optics 34, as shown. Other designs can employ other configurations.
In an alternate design, the throat portion 16 can be vibrated by a suitable device, such as a piezoelectric vibrator, to cause the liquid target material being emitted therefrom to form a stream of droplets. The frequency of the agitation determines the size and spacing of the droplets. If the target stream 18 is a series of droplets, the laser beam 22 may be pulsed to impinge every droplet, or every certain number of droplets.
It is desirable that as much of the EUV radiation as possible be collected to improve source efficiency. For example, the higher the intensity of the EUV radiation for a particular photolithography process, the less time is necessary to properly expose the various photoresists and the like that are being patterned. By decreasing the exposure time, more circuits can be fabricated, thus increasing the throughput efficiency and decreasing the cost. Further, by providing more useable EUV radiation from the collector optics, the intensity of the laser beam can be lower, also conserving system resources.
Optimizing the reflectance of the reflective surface of the collector optics is one way in which the amount of the EUV radiation that is collected can be increased. Typically, the reflective surface of the collector optics is coated with a reflective coating to enhance its reflectance. However, it is also important that the coating material not contaminate source components in response to high energy ions generated by the plasma that may impinge the reflective surface and release coating material. One such coating that provides the desired characteristics is silicon/molybdenum (Si/Mo) multilayer. However, the best Si/Mo coating on the collector optics only reflects about 70% of the EUV radiation impinging thereon, even at its theoretical maximum performance.
FIG. 2 is a plan view of collector optics 40 including an elliptical reflector 42 that collects and focuses EUV radiation 44, and is the same type as the collector optics 34 in the source 10. The elliptical reflector 42 includes a center opening 46 through which the laser beam propagates to a target area 50 at a focal point of the reflector 42 to generate the EUV radiation 44 in the manner as discussed above. The reflector 42 has an elliptical shape in that if the lines representing the reflector 42 were continued they would form a figure of rotation with elliptical cross section. The complete reflector 42 extends into and out of the paper to form an ellipsoidal reflector dish, as is well understood in the art.
As discussed above, it is important to collect as much of the EUV radiation 44 as possible. In the design shown herein, the EUV radiation 44 is reflected off an inner surface 58 of the reflector 42 and is directed to the other focal point 52 of the ellipse. Outer rays 54 and 56 of the EUV radiation 44 represent the outer most location relative to an outer edge 62 of the reflector 42 that the EUV radiation 44 is able to be collected, and still be directed towards the focal point 52. The outer most rays 54 and 56 define an outer surface of a cone within which the EUV radiation 44 is usable. Any EUV radiation outside of the cone defined by the rays 54 and 56 will not be useable because it is outside of the allowed collection angle of the photolithography side of the system that may use the EUV radiation source. Therefore, the size of the reflector 42 is limited, as shown. However, some of the EUV radiation 44 emitted from the target area 50, represented here as rays 60, is not collected by the reflector 42, and is thus lost in the system as wasted EUV radiation. The reflector 42 cannot be made any larger to collect the rays 60, and still satisfy the angular collection requirement.